What does an MRI scanner use to generate images?

• A. Strong magnetic fields, magnetic field gradients, and radio waves to generate images
• B. Ionizing radiation from X-rays to generate images
• C. Electrical impulses to stimulate tissues
• D. Sound waves to measure tissue density

Answer: (A)

MRI images come from a combination of a very strong static magnetic field, magnetic field gradients, and radiofrequency energy. The strong field aligns the spins of hydrogen nuclei in the body. A radiofrequency pulse tilts these spins away from alignment, and as they relax back, they emit signals that are captured by detectors. The spatial gradients in the magnetic field allow those signals to be mapped to specific locations, so the data can be reconstructed into detailed images. This approach uses non-ionizing energy, and it specifically exploits differences in how tissues respond to the magnetic field and radiofrequency pulses.

The other options describe imaging methods that aren’t how MRI works: X-ray–based imaging uses ionizing radiation; electrical impulses to tissues are a stimulation technique, not an imaging method; sound waves used in ultrasound image tissue by measuring echo characteristics, not through magnetic fields or radiofrequency signals.